<p>The preservation and energy-efficient renovation of historical buildings poses significant challenges owing to legal and structural constraints. In Korea, where historic façades are protected under national heritage regulations, internal insulation is one of the few viable retrofit options. Seven commercially available insulation materials—expanded polystyrene (EPS), extruded polystyrene (XPS), polyurethane (PU), phenolic foam (PF), glass wool (GW), mineral wool (MW), and vacuum insulated panel (VIP)—were evaluated based on measured thermal conductivity and applied to the internal walls of a 1924 historic building in Seoul. Thermal performance simulations using actual material properties confirmed that all materials, when applied at 120&#xa0;mm thickness, met the national U-value requirement of 0.240 W/m²·K, with VIP and PF offering notably superior performance. Building energy simulations showed that heating energy consumption was reduced by 11–16% in winter and cooling energy use by 3–7% in summer. Indoor temperature measurements supported improved thermal comfort following insulation retrofitting. Economic analysis revealed that material price was a more decisive factor than performance in determining cost-effectiveness, with PF and EPS demonstrating the most favorable payback periods and net present values. By combining experimental data with simulation-based analysis, the study offers practical guidance for balancing energy performance and heritage conservation in masonry buildings.</p>